Fix PHP fatal when running WP-CLI

[tbradsha]

When running something like wp site empty on WP Cloud sites, it gives a PHP fatal.

This seems to have been introduced in 123e2a0 (see #44772).

After much trial and error, I found that deferring the schema generation to a callback seems to work fine, but I'm not familiar with endpoints or the implications of making this change, and have no idea why it works, particularly because effectively the exact same thing happens in the args key.

Proposed changes:

Other information:

• Have you written new tests for your changes, if applicable?
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Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

wp site empty should work fine with this branch, but I'm not sure how to test for regressions.

[github-actions]

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[github-actions]

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Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/my-jetpack/src/class-rest-products.php	158/256 (61.72%)	-6.91%	18 💔

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}

[grzegorz-cp]

Lis looks good to me.
I also tested the scenarios from #44772